Two plants near Tokyo, each with multiple reactors are on the verge of meltdown after emergency backup cooling was shut down by loss of power due to flooding. (Source: CNN)

An explosion damaged the roof of one plant, releasing radiation on Saturday. (Source: Reuters)

The plants lie within the Tokyo metropolis. People are being evacuated from within a 20 km radius. (Source: CNN)

Japanese nuclear disaster is cause for pause, reflection

The Sendai Earthquake struck Japan early Friday morning with unrelenting fury. Measuring 8.9 to 9.1 Mw-megathrust the quake was among the five most severe in recorded history and the worst quake to hit Japan. In the aftermath of this severe disaster, as the nation searches for survivors and contemplates rebuilding, an intriguing and alarming storyline has emerged -- the crisis at the Fukushima Daiichi nuclear plant.

You may recall that a few years back Japan was struck by another quake which cracked the concrete foundation of a nuclear plant, but yielded virtually no damage.

By contrast, this time the damage was far worse, creating what could legitimately be called a nuclear disaster.

I. Fukushima Daiichi - a Veteran Installation

The Tokyo district of Fukushima is home to two major nuclear power installations.

In the north there is the Fukushima "Daini" II plant, which features four reactors -- the first of which went online in 1982. These units produce a maximum of 4.4 GW of power and are operated by the Tokyo Electric Power Company.

To the south lies the Fukushima "Daiichi" I plant, a larger and older installation featuring six reactors, the first of which went online in 1970. Operated by Tepco, the installation offers a combined 4.7 GW of power. It was here that disaster struck.

II. Disaster at Fukushima Daiichi 1

While the southern installation is over four decades old, Japan has been responsible in retrofitting the plant with modern safeguards. Among those is an automatic switch which shuts off the reactor when an earthquake struck.

The switch performed perfectly when the quake hit Friday morning, shutting of the three reactors that were active at the time. Control rods lowered and the reaction stopped.

The next step was the cooling the power rods, composed of uranium-235, to prevent them from melting.

Cooling water was pumped over the rods for about an hour, but before the rods could be fully cooled, stopping the reaction, the pumps failed. According to the International Atomic Energy Agency, and multinational oversight group, the failure was due to failure in the backup generators due to the tsunami flooding.

On Saturday Japanese authorities and power officials tried to use sea-water injections to complete the cooling process, but those plans were stalled when another tsunami warning arrived.

An explosion occurred inside at least one of the reactor buildings. It is believed to be due to the build-up of pressure after the pumps failed creating hydrogen and oxygen gases, which subsequently combusted from the heat.

Malcolm Grimston, Associate Fellow for Energy, Environment and Development at London's Chatham House toldCNN:

Because they lost power to the water cooling system, they needed to vent the pressure that's building up inside.My suspicion is that as the temperature inside the reactor was rising, some of the metal cans that surround the fuel may have burst and at high temperature, that fuel cladding can react with water to produce zirconium oxide and hydrogen.That hydrogen then will be part of the gases that need to be vented. That hydrogen then mixes with the surrounding air. Hydrogen and oxygen can then recombine explosively. So it seems while the explosion wasn't directly connected with the nuclear processes, it was indirectly connected, because the hydrogen was only present because of what was going on in the reactor core.

The explosion damaged the roof of the plant and sent billows of smoke up into the air. According to officials some radioactive material was released into the atmosphere. Outside the plant perimeter, levels of radiation measured 8 times higher than normal.

Meanwhile reactors at the newer Fukushima II are also beginning to heat up after their own cooling systems failed.

Japanese officials have evacuated people from an expanding radius around the plants as a precaution. Currently the evacuation zone is at almost 20 km. They hope to try to continue cooling, but have to work around tsunami alarms from earthquake aftershocks that have continued into Saturday. U.S. Secretary of State Hillary Clinton has announced that the U.S. is sending high-tech coolant to the plants, in and attempt to avert disaster.

III. Can a Meltdown be Avoided?

Without proper cooling, the rods will continue to heat and proceed towards meltdown, releasing clouds of radioactive gas. The first question is thus whether meltdown can be avoided.

At the Fukushima I plant, radioactive cesium was discovered. Cesium is in the beta decay chain tellurium -> iodine -> xenon -> cesium. Its occurs roughly 16 hours after an unchecked uranium reaction and its presence indicates that one of the fuel rods may already have melted down.

Once one rod melts, it will be much more difficult to prevent the others from melting down as well.

According to reports, the coolant temperatures inside the reactor have exceeded 100 degrees Celsius. If they reach 540 degrees Celsius the fuel rods will fully melt down.

The question now becomes what to do.

According to reports by Nippon Hoso Kyokai (Japan Broadcasting Corporation), three individuals have already been exposed been the victims of radiation poisoning (likely plant workers) and that radioactivity levels at the plant have risen to 1,000 times the normal levels at the plant control room.

One option on the table is to vent the reactors, allowing them to blow off the steam and prevent a greater buildup of pressure and heat. However, doing so could release significant levels of radioactivity into the surrounding area.

The alternative is to try last ditch cooling and hope that if the rods do melt, that the secondary containment will hold. The release of radioactive gases from venting would pale to that if the secondary containment was breached. Such a scenario would likely result in the modern day equivalent of Chernobyl.

Whichever course of action is selected, there's a great deal of risk of radiation exposure to those who inhabit the area in the near future. States James Acton, international physicist, in an interview with CNN, "There's a possibility of cancer in the long term -- that's the main hazard here."

IV. Grim Lessons From the Disaster

At Three Mile Island, the U.S. learned the hard way not to put vital controls in the hands of plant operators. Operators almost created a meltdown, when they accidentally disabled necessary cooling. That was due to the poor quality of indicators.

As the result, the nuclear community learned to automate shutdown processes.

Ultimately the Fukushima disaster illustrates the need for sealed backup generators. The containment procedures in all their modern glory are useless if the backup power goes out. And, if possible, it shows that it is desirable to build new nuclear plants farther from the sea and from fault lines (though this could cause costs to increase).

As the fight to avert meltdown plays out, the final damage won't be known for weeks to come. But the international community is already reacting.

At this time it's vital not to overreact to this worse case scenario.

The disaster does illustrate that nuclear fission power is far from failsafe, particularly older reactors -- even if retrofitted with modern controls. Ultimately the international community needs to work towards fusion power, which should be much safer and cheaper.

At the same time, it's important to consider that there's a great deal of background radiation released from the burning of fossilized coal and that mining fossil fuels has led to many a great loss of life and resources as illustrated by recent coal and oil disasters.

There's no easy answers here. Oil and coal power emit dangerous nitrogen and sulfur-containing gases and carbon dioxide into the ozone. And their fuel is dangerous to obtain. But they're cheap. Solar and wind power are relatively safe, but they're expensive and offer inconsistent power. Nuclear power is cheap and produces no emissions normally, but it can be a danger in the case of natural disaster or malicious attack.

It's important not to turn a blind eye to this disaster, but it's equally important not to overreact.

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"Ultimately the Fukushima disaster illustrates the need for sealed backup generators. The containment procedures in all their modern glory are useless if the backup power goes out. And, if possible, it shows that it is desirable to build new nuclear plants farther from the sea and from fault lines (though this could cause costs to increase)."

I strongly disagree. Ultimately, the Fukushima disaster illustrates the need to progress to modern reactor designs such as the German pebble bed AVR, the Chinese HTR-10 or Thorium reactors, as examples. None of these types require emergency cooling. There have been many other tested designs in the past fifty years. Thorium reactor technology, in particular, has been around for almost as long as nuclear power.

Progress toward safe reactors can only come with cooperation from governments, and cooperation from governments will come only with education and pressure from the public. And pressure from the public can only be generated by a well-educated press. That means you, Jason!

quote: I strongly disagree. Ultimately, the Fukushima disaster illustrates the need to progress to modern reactor designs such as the German pebble bed AVR, the Chinese HTR-10 or Thorium reactors, as examples.

You forgot to mention that the CANDU designs which would have simply returned to an inert state once the heavy water moderator was removed and that all of the backup and safety systems are powered by gravity and back-pressure. Remove the back-pressure and gravity will drain the moderator and stop the reaction.

Light water reactors such as Fukushima are inherently dangerous as the fuel rods have to be brought down from a higher energy state to achieve a sustainable chain reaction. This is dangerous. All of the designs mentioned by Voronwe and myself require some sort of 'trick' to provide the environment necessary for a sustainable chain reaction to continue. Remove the moderator and suddenly everything stops. Additionally, most of them are built around safety systems that don't even require external power in order to function, such as gravity powered evacuation and cooling mechanisms. Some can even be "poisoned out" by pouring neutron inhibitors through the fuel rods themselves, stopping even the most runaway reaction dead in it's tracks.

That comment I made was exclusively w.r.t. traditional modern uranium reactor designs.

Thorium designs are promising, but produce less power per gram of fuel than uranium designs making them a tough sell.

As to the pebble bed design (like China's HTR-10) I think it's great -- generally very safe and efficient. However, it could actually be more dangerous in a case like this. If the quake was able to crack the inert gas containment vessel and oxygen got in, the pebbles could burn due to the high carbon content, particularly if some pebbles cracked compromising the silicon carbide layer. This burning could lead to similar radioactive gas release.

Of course proper containment would likely go a long way to preventing this, but that speaks back to my original point -- that you need to take lessons learned from incidents like this and incorporate them into a robust containment scheme.

And even every new reactor made was Thorium, there'd still be plenty of operational uranium legacy designs to apply those lessons on. Building advanced designs won't shutdown legacy reactors overnight. New safeguards on these reactors is obviously critical (nuclear pun not intended!).

quote: Thorium designs are promising, but produce less power per gram of fuel than uranium designs making them a tough sell.

Please explain.

quote: As to the pebble bed design...

Why is it that you believe that it's more difficult to contain a pebble bed design than currently extant types of reactors? And given that the problems a pebble bed addresses are precisely the ones that threaten containment in the current Japanese situation, why would it be more dangerous in a case like this?

What new safeguards on legacy reactors do you suggest would solve the problems the Japanese face?

Well there is plenty of thorium, but due to the lower energy density you'd need to build a larger reactor/react more material.

I'm not saying thorium designs are a bad idea, but the technology has never been tested on a commercial-scale LWR, so we don't really have a good idea whether it's a suitable commercial replacement yet.

Ultimately I think CANDU pressurized HWRs like Canada's ACR-1000 project are more promising. They produce a lot of energy, are safer, and can use a wider variety of nuclear materials like LWR waste, decomissioned warhead material, and specialized fuel.

quote: Why is it that you believe that it's more difficult to contain a pebble bed design than currently extant types of reactors? And given that the problems a pebble bed addresses are precisely the ones that threaten containment in the current Japanese situation, why would it be more dangerous in a case like this?

As I stated, the danger comes from the carbon that acts as a neutron moderator. Carbon is flammable at high temperatures. Thus any carbon moderator design poses a risk of flammability.

In the case of the pebble bed, an inert gas is used inside the reactor core to prevent oxygen from reaching the carbon (combustion reactions are oxidative). Further a silicon carbide layer is applied to the pebbles to further decrease flammability. Unfortunately, both safeguards can be directly compromised by physical damage -- such as the physical stress from an earthquake.

Most existing pebble designs are extremely vulnerable to shear stresses -- such as be jarred against each other during a quake.

Once cracked and the inert gas containment is compromised, the carbon is likely to burn releasing radioactive gases.

quote: What new safeguards on legacy reactors do you suggest would solve the problems the Japanese face?

Simple. Include sealed backup generators. This should be a relatively simple task, though it would likely cost a bit of extra money. Even if the line was broken, then, it could quickly be restored if backup power lines were kept available on site.

The net result is that the coolant pumps would be powered and virtually any possible risk of legacy reactors in terms of natural disaster would be averted.

Again I'm all for researching pebble bed and thorium designs, but I think they're unproven in terms of commercialization -- particularly thorium ones.

I think CANDU heavy water reactors are attractive from a variety of perspectives, including safety and fuel availability. AND they are commercially proven, having been used for decades in Canada and elsewhere.